CAVES, or CAVERNS (Lat. earns, hollow), are, hollow places in the earth. They are either natural or artificial. Natural C. have been produced by the fracture and disloca tion consequent on the upheavral of the strata, by water, or by both causes combined. The denuding or eroding power of water, which has produced the materials of stratified rocks, has formed caverns in the courses of rivers and on the coast-line of the sea. The moving waters, carrying with them gravel and sand, enter natural cracks and crevices, and, by abrading their walls, increase their size, so as to form C.; or they attack less indurated portions of the solid strata, and form cavities bounded by the harder rock. Such caverns are of frequent occurrence round the shores of Britain, and indicate, where they occur, an ancient sea-margin of the island. In limestone rocks, the destroy ing power of water is increased when it contains chemical agents which have the power of dissolving the substance of the rock, and so causing it to he carrried off in solution by the water.
C. most frequently occur in limestone rocks. They especially abound in the oolitic limestone, which on this account was called by the earlier continental geologists "cav ern limestone." The celebrated C. of Franconia in Germany. of Kentucky (q.v.) in America, that of Kirkdale in Yorkshire, and many others, occur in this formation. Next to limestone, thelriassic measures, containing roc] salt, a material, easily removed by water, most abound in caverns. They are also frequently met with in igneous rocks —the picturesque cave of Fingal, in Staffs, is formed in basalt; and in South America and Iceland the modern lava contains huge caverns Many caverns have a calcareous incrustation lining their interior, giving them a gor geous appearance. Sometimes this deposit is pure white, and has, when the cave is lighted up, a richness and. transparency that cannot be imagined. It is, however, more generally colored by the impurities which the water has taken up from the superineum bent strata. To the incrustations which are suspended from the roof, like icicles, the
name stalactite is given, while those rising from the floor are designated stalagmites. Sometimes the pendent stalactite is produced so as to meet the ascending stalagmite, and form pillars, as if to support the roof, as in the "organ" in the Blue John Mine, Derbyshire. The source and origin of this deposit has been satisfactorily explained by Liebig as follows: The mold of the superficial soil, being acted upon by moisture and air, evolves carbonic acid, which is dissolved by rain. The rain-water thus impregnated, permeating the calcareous strata, has the power of taking up a portion of the lime, which it retains in a liquid condition, until from evaporation the excess of carbonic acid is parted with, when the lime again returns to its solid state, and forms the incrustation.
C. have an additional interest to the geologist, from the occurrence in many of osseous remains under the calcareous incrustations of the floor. The bones are imbedded in mud, and frequently concreted into a firm calcareous breccia. They belong to the pleie tocene period, when the C. in Europe were inhabited by large hyenas and bears. Por tions of other animals inhabiting the neighborhood were dragged by them into their dens, to serve as food. In this way the bones of herbivorous and other animals are found mixed with those of the beasts of prey; they have a broken and gnawed appearance, similar to what is produced on recent bones by the teeth of a hyena. No less than 33 species of mammals and 5 species of birds have been discovered in the C. of the British islands, of which about the half still survive in Europe, while the remainder are extinct. The mammals are species of ox, deer, horse, wolf, dog, hare, fox, weasel, water-rat, mole, bat, hippopotamus, and rhinoceros, besides the hyena and bear; and the birds are species of lark, partridge, pigeon, goose, and crow.